The present invention refers to a system and a method of automatic defrost to be applied to refrigeration appliances, such as refrigerators and freezers.
The refrigeration appliances of the natural convection type usually do not execute defrost automatically, requiring the user to promote the defrost operation manually, by turning off the refrigeration appliance (or only the compressor thereof) during the time necessary to melt all the ice accumulated on the evaporator.
The disadvantage of this system resides in the fact that the user has to turn off the refrigeration appliance manually.
In another construction of a refrigeration appliance, the thermostat of the latter is provided with a knob, for turning off the compressor, which will turn on again only when the temperature sensed by the sensing bulb of said thermostat reaches a predetermined temperature, sufficiently hot to melt the ice on the evaporator. This concept is used in most refrigeration appliances provided with a cabinet door. In this case, the user has to turn off the compressor every time a defrost operation of the evaporator is needed. Besides the disadvantage of requiring the user to initiate each defrost procedure, in this solution the refrigeration appliance loses almost all its function of promoting refrigeration during the defrost operation. The refrigeration appliances known as xe2x80x9ccycle defrostxe2x80x9d have another constructive variant for the defrost system. In these refrigeration appliances, the thermostat has a compressor turn on temperature, which remains fixed, regardless of the adjustment applied to the thermostat. Thus, every time the sensing bulb of the thermostat reaches a compressor turn off temperature, said compressor will turn on only when said sensing bulb detects said compressor turn on temperature. If this temperature is dimensioned to be positive and has a previously determined value, in order to allow the melting of the ice accumulated on the evaporator, the refrigeration appliance will promote defrost of the evaporator at each cycle of the compressor, whereby the user does not have to worry about executing this task.
Although this solution does not present the disadvantages of the previous solutions, it allows the occurrence of a great temperature variation over the food articles stored in the refrigeration appliance, which variation is much greater than in the previous cases, jeopardizing the preservation of said food articles.
Moreover, in load conditions or also in determined environmental conditions in the place where the refrigeration appliance is installed, the compressor may not turn off, and thereby will not promote the defrost operation. In these cases, after a certain time of refrigeration operation, there will be an excess of ice formed in the region of the evaporator that will act as a thermal insulator, impairing the refrigeration efficiency of the refrigeration appliance. On the other hand, in a normal operational condition of the refrigeration appliance, the defrost system of the latter will promote defrost at each cycle of the compressor, even when this operation is not required, causing unnecessary energy consumption, impairing the thermal recovery of the refrigeration order to count each first and second turn on conditions of the electric motor, the control unit selectively activating the thermal switch means to operate: a- in the first turn on condition and in the turn off condition, while the period of time in the first turn on condition is inferior to a respective predetermined maximum safety value and the number of cycles in the first turn on condition of the electric motor is inferior to a predetermined number; b- in the turn off condition and in the second turn on condition, when the predetermined number of cycles of the first turn on condition of the electric motor has been reached and until a subsequent second turn on condition has been reached; c- in the first turn on condition and in the turn off condition, after the second turn on condition has been reached, if the period of time in the second turn on condition is inferior to a respective predetermined maximum safety value, the control unit instructing the switch means to interrupt the energization of the electric motor when the timer indicates to the control unit a period of time of any of the first and second turn on conditions of the electric motor which is equal to the respective predetermined maximum safety values and maintaining said switch means condition until the occurrence of a subsequent second turn on condition of the electric motor, when the control unit instructs the switch means to allow the energization of the electric motor, the control unit activating the thermal switch means to operate with the other turn off condition, after the second turn on condition has been reached, if the period of time in one of the preceding turn off conditions and another turn off condition is equal or superior to a respective predetermined maximum defrost value, and to operate with the turn off condition and the first turn on condition, after the second turn on condition has been reached, if the period of time in the other preceding turn off condition is equal or inferior to a predetermined minimum defrost value.